High efficiency light collection is important in a number of applications, including lighting and illumination, displays, document scanning and machine vision, signalling, aviation and automotive lighting, medical instrumentation, infrared and optical wireless communications, and signal detection. Typically a light collection optical system is needed to convert a first spatial and angular distribution to a second, different spatial and angular distribution. Very commonly the collector couples light from a small, wide-angle source to a larger more collimated beam. It is generally desirable that such light collectors couple the highest possible fraction of light into the desired aperture and angles, with minimum size and cost.
Various light collectors are known in the art. Spherical lenses, aspheric lenses, and combinations of parabolic, elliptical, and hyperbolic mirrors have been used for centuries. Most of these systems are “imaging,” meaning that the surfaces are designed to redirect light from a central point or angle in the first distribution to a central point or angle in the second distribution. Light from points or angles near enough to the center point or angle in the first distribution is, by similarity, redirected into the neighborhood of the center point or angle in the second distribution, with the same number of reflections or refractions for almost all the rays of interest. Unlike the central rays, the non-central points and angles are only approximately redirected into each other. Therefore control over the edges of the distributions is typically limited, and one of the light distributions often spreads over larger areas or angles than is desired, with non-uniform beam output and gradual rather than sharp angular cut-off. Control is particularly limited when one of the distributions has very large angles, or when the spatial extent of the smaller distribution is not much smaller than a characteristic length scale of the collector.
More recently introduced are “edge-ray” collectors which are designed to redirect the rays at the spatial or angular boundary of the first distribution to a spatial or angular boundary of the second distribution. It can be shown that, when distribution boundaries are so coupled, the rays in the interior of one distribution will then be coupled into the interior of the other distribution. However, different portions of the interior typically have a different number of reflections or refractions from each other or from the edge. In undergoing these different numbers of reflections or refractions, adjacent portions of the first distribution may end up non-adjacent in the second distribution, and therefore these collectors are “non-imaging.” These non-imaging collectors provide much more precise control over the spread of the light distributions, typically maintaining both distributions within their theoretical limits even for large-area or large-angle beams that are poorly handled by imaging collectors. This more precise control is often desirable for the applications described above. Typically for these collectors opposite surfaces are designed to redirect opposite edges of the distribution.
Simple imaging collectors are typically very compact: for example, a parabolic mirror with ±90° light collection has a length-to-diameter ratio (“aspect ratio”) of 0.25. By comparison, many non-imaging designs are undesirably U.S. Pat. No. 4,240,692 describes a non-imaging concentrator known as a Compound Parabolic Concentrator (CPC). The CPC is a hollow, funnel-shaped, mirror that redirects rays from a spatial edge at its small end into the angular edge of a beam at its large end. For narrow-angle beams, the CPC is undesirably long: for example, the aspect ratio of a ±10° CPC is over 3. The CPC can be truncated to reduce the length, but then efficiency is reduced or the spread of the light distribution is increased.
This aspect ratio has been reduced by a class of collectors using one refractive surface with a funnel-shaped reflective light-pipe. For example, U.S. Pat. No. 4,114,592 shows an alternate edge-ray collector known as a Dielectric Total Internal Reflection Concentrator (DTIRC) that uses a spherical refracting front surface. This improvement reduces the aspect ratio of a ±10° collector to approximately 1.7. U.S. Pat. No. 5,285,318 improves on the DTIRC by using an aspheric instead of a spherical refracting surface, reducing the ±10° aspect ratio to about 1.3. Friedman and Gordon published a further improvement in “Optical designs for ultrahigh-flux infrared and solar energy collection: monolithic dielectric tailored edge-ray concentrators,” Applied Optics, Vol. 35, No. 34, 1 Dec. 1996, pp. 6684–91. They showed that with a different aspheric refracting surface the ±10° aspect ratio could be reduced to about 1.2, and that this was the theoretical limit with a single refraction at the front surface. Moreover, these designs require very thick dielectric components, which are difficult to mold accurately at low cost.
Minano and co-workers have published several designs that combine one refractive surface and one or more reflective surfaces. These designs reduce the aspect ratio to approximately 0.25; but in all these designs the small aperture is placed in front of a large back-reflecting mirror, so that the small aperture obstructs the large aperture. When the apertures are very different in size, as for narrow-angle collimators, the area ratio is low; and the obstruction can be small, but for larger angles the obstruction is unacceptable. Moreover, these collectors are often undesirable when a source or detector at the small aperture needs to be supported by a substrate including a circuit board or heat sink, as is common with high power LED light sources, for example. Minano and co-workers have also published designs with two refracting surfaces and no reflecting surfaces, but the largest collection angle at the small aperture is limited.
The current invention uses an aspheric dielectric lens with two refracting surfaces at the large aperture of a hollow, funnel-shaped reflector. The back surface of the dielectric (the surface facing the reflector) has higher curvature than the front surface, making the structure more compact. This approach achieves performance comparable to a non-truncated CPC, with much better compactness. Aspect ratios range from 0.4–0.75. Moreover, the dielectric lens has acceptably low thickness for cost-effective molding. Unlike the Minano designs, the small aperture of the funnel is advantageously positioned behind the optic, so that a source or detector can be supported by a much larger circuit board or heat sink without shadowing. Winston and co-workers have published designs with a spherical lens and funnel-shaped reflector, including U.S. Pat. No. 5,243,459, but these designs are not nearly as compact as the current invention.